Add test case for normal merge

* tests/normal_merge.R: Test number of parents and commits of a normal
  merge.

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Fix parents of a normal merge

* src/git2r_merge.c (git2r_retrieve_parents): Add function to retrieve
  the parents of the merge commit from the MERGE_HEAD file.

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Remove metadata after merge commit

* src/git2r_merge.c (git2r_normal_merge): Call
  git_repository_state_cleanup to remove metadata associated with
  merge commit.

* tests/normal_merge.R: Add test to check cleanup.

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Fix compiler warning; unused variable

* src/git2r_merge.c (git2r_retrieve_parents): Remove unused variable
  sha.

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>